Gas in the Universe went from being mostly neutral to mostly ionized as the first galaxies formed, and the signature of this process is imprinted in quasar spectra. The review of the classic paper by Gunn & Peterson continues in this second in the three-part series.

This guest post, the first in a three-part series, reviews the classic article by Gunn & Peterson (1965). This paper proposed several fundamental ideas in cosmology, including using distant quasars as “flashlights” to observe the diffuse gas between galaxies.